Help: Rework $<LINK_LIBRARY>, $<LINK_GROUP> and related docs

These changes restructure the docs to improve readability and flow, correct grammar and typos, and fix errors and inconsistencies in some of the examples. Most of the information was there in the original docs, but I found it difficult to follow and understand the concepts. I've reorganised the content to make it flow in a more progressive manner, use consistent patterns and reduce disruptive elements that were not really needed (e.g. quite a few .. note:: directives that mostly were better absorbed into the main text).

Backport: release